Global overpopulation would ‘withstand war, disasters and disease’.

National Academy of Sciences says even brutal world conflict or lethal pandemic would leave unsustainable human numbers.

People hang onto a crowded train as they travel to Colombo. Sri Lanka. The report says south Asia and Africa will be the hardest hit by population growth.Photograph: Dinuka Liyanawatte/Reuters

The pace of population growth is so quick that even draconian restrictions of childbirth, pandemics or a third world war would still leave the world with too many people for the planet to sustain, according to a study.

“We were surprised that a five-year WW3 scenario, mimicking the same proportion of people killed in the first and second world wars combined, barely registered a blip on the human population trajectory this century,” said Prof Barry Brook, who co-led the study at the University of Adelaide, in Australia.

The second world war claimed between 50 million and 85 million military and civilian lives, according to different estimates, making it the most lethal conflict, by absolute numbers, in human history. More than 37 million people are thought to have died in the first world war.

Using a computer model based on demographic data from the World Health Organisation and the US Census Bureau, the researchers investigated different population reduction scenarios. They found that under current conditions of fertility, mortality and mother’s average age at first childbirth, global population was likely to grow from 7 billion in 2013 to 10.4 billion by 2100.

Climate change, war, reduced mortality and fertility, and increased maternal age altered this prediction only slightly. A devastating global pandemic that killed 2 billion people was only projected to reduce population size to 8.4 billion, while 6 billion deaths brought it down to 5.1 billion.

“Global population has risen so fast over the past century that roughly 14% of all the human beings that have ever existed are still alive today. That’s a sobering statistic. This is considered unsustainable for a range of reasons, not least being able to feed everyone as well as the impact on the climate and environment,” said co-author Prof Corey Bradshaw, also from the University of Adelaide.

Crowds at Oshidin market in Lagos, Nigeria.Photograph: Alamy

He added: “We examined various scenarios for global human population change to the year 2100 by adjusting fertility and mortality rates to determine the plausible range of population sizes at the end of this century. Even a worldwide one-child policy like China’s, implemented over the coming century, or catastrophic mortality events like global conflict or a disease pandemic, would still likely result in 5-10 billion people by 2100.”

Brook, now at the University of Tasmania, said policymakers needed to discuss population growth more, but warned that the inexorable momentum of the global human population ruled out any demographic quick fixes to our sustainability problems.

“Our work reveals that effective family planning and reproduction education worldwide have great potential to constrain the size of the human population and alleviate pressure on resource availability over the longer term,” he said. “Our great-great-great-great grandchildren might ultimately benefit from such planning, but people alive today will not.”

Bradshaw added: “The corollary of these findings is that society’s efforts towards sustainability would be directed more productively towards reducing our impact as much as possible through technological and social innovation.”

With its echoes of Thomas Malthus, who warned of the unsustainability of rapid population growth in the 18th century, the report warned that the current demographic momentum means that there are no easy policies to change the size of the human population substantially over coming decades, short of extreme and rapid reductions in female fertility.

“It will take centuries, and the long-term target remains unclear,” said the report. “However, some reduction could be achieved by mid-century and lead to hundreds of millions fewer people to feed. More immediate results for sustainability would emerge from policies and technologies that reverse rising consumption of natural resources.”

In the absence of catastrophe or large fertility reductions (to fewer than two children per female worldwide), the study said Africa and south Asia are likely to experience the greatest human pressures on future ecosystems.